Carrier Aggregation will be used in 3GPP LTE networks to provide improved data rates to users. Carrier aggregation consists of transmitting data to or receiving data from the UE on multiple carrier frequencies referred to herein as “component carriers” or “carriers”. The wider bandwidth enables higher data rates.
The present disclosure considers the problem of performing handover of a mobile station from a source base station (also known as source node-B, source evolved-node-B, or source eNB) to a target base station (also known as target node-B, target evolved-node-B or target eNB) when Carrier aggregation (CA) is in use. The currently specified LTE handover procedure supports handover of a mobile station while operating on a single carrier (i.e. in the absence of carrier aggregation).
For carrier aggregation (CA) operation, a mobile station (also referred to as user equipment or “UE”) can be configured with a set of component carrier (CCs). Component carriers can be downlink (DL) component carriers (used for transmission from an eNB to UEs) or uplink (UL) component carriers (used for transmission from UEs to eNBs). Typically, each uplink CC has a corresponding downlink CC to which it is paired. The pairing normally ensures that if the UE transmits a random access preamble on an uplink CC, the response message to the random access preamble transmission is received on the paired downlink CC. There can be situations where a downlink CC is configured but the paired uplink CC is not configured, and vice versa. Some of the configured CCs may be activated. The activated CCs can be used to send and receive data (i.e., the activated CCs can be used for scheduling) to and from the UE. The UE has up to date system information for all configured CCs or at least the configured CCs that the network expects to activate. Therefore, after a CC has been configured, it can be quickly activated, without experiencing the delay due to delivery of relevant system information needed to perform communication on such a CC. Thus, when there is a need for aggregating multiple CCs (e.g., a large burst of data), the network can activate configured CCs. The maintenance of a configured CC set in addition to an activated CC set enables battery conservation in the UE by ensuring that CCs need to be activated only when there is a substantial amount of data to be transmitted.
The currently specified LTE handover procedure requires the network to first bring the set of activated CCs down to a single CC before the handover. The network can re-activate CCs as needed after the handover is completed. The problem with such a handover procedure is that when carrier aggregation is being used, it can cause significant inefficiencies and delays in the data transfer. As mentioned above, carrier aggregation is expected to be used (i.e., multiple CCs are expected to be activated) only when there is a substantial amount of data to be transmitted). If a handover occurs during such a data transfer, this option would result in buffering of large quantities of data at the source eNB and subsequent transfer of the data to the target eNB (and possibly also buffering of large quantities of data at the UE). It can also cause excessive buffering of data at the target eNB as re-activation of CCs at the target eNB can take significant time after the handover.
Therefore it is beneficial to have handover mechanisms that are more suitable for carrier aggregation operation.